1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus that suppresses occurrence of color shift in images and a method of controlling the same.
2. Description of the Related Art
In a tandem-type electrophotographic image forming apparatus including photosensitive drums, transfer positions at which images are primarily transferred from the respective photosensitive drums onto an intermediate transfer member are different between the photosensitive drums. Therefore, if the timing of starting to form an electrostatic latent image on each photosensitive drum is not adjusted to be uniform between the colors, the transferred images formed on the intermediate transfer member are displaced between the colors, causing image color shift, which is positional displacement between images of respective colors formed on a recording sheet. To prevent this problem, it is necessary to cause positions of the images transferred onto the intermediate transfer member to be adjusted to be uniform by shifting the timing of starting to form an electrostatic latent image between the photosensitive drums by a time required for an image already transferred onto the intermediate transfer member to be conveyed from one photosensitive drum to the next one.
Further, the photosensitive drums and the intermediate transfer member are required to be driven at a constant surface speed.
This is because, first, in a case where time-synchronized exposure is employed as laser exposure for drawing an electrostatic latent image on the photosensitive drum, variation in the surface speed of the photosensitive drum causes deviation of a laser-irradiated position on the photosensitive drum from an original proper position to be irradiated.
Secondly, also in a primary transfer process for transferring a toner image formed on the photosensitive drum onto the intermediate transfer member, if there occurs an AC current-like variation in the difference of surface speed between the photosensitive drum and the intermediate transfer member, the position of the toner image which is to be transferred onto the intermediate transfer member deviates from the original proper position on which the toner image is to be transferred.
To overcome the above-mentioned problem, in the control of driving the photosensitive drum, the feedback-control of the speed of a motor as a drive source is performed, using various speed detection sensors and the like, whereby highly-accurate speed constancy is ensured. Further, as the motor, one employing a brushless DC motor (hereinafter referred to as the “BLDC motor”) is often used because of low-cost, quietness, and high efficiency.
In recent years, for the speed feedback control using the BLDC motor, a method is sometimes employed in which a rotary encoder is arranged on a drum shaft, and the CPU controls the BLDC motor to rotate the drum shaft at a constant speed.
In the above-mentioned speed feedback control, the rotational speed of the drum shaft is detected, but it is difficult to control the surface speed of the photosensitive drum to a constant speed due to off-centering, a mounting error and the like of each of the photosensitive drum, the motor, and drive gears.
Similarly, in the intermediate transfer member as well, it is difficult to control the surface speed of the intermediate transfer member to a constant speed due to off-centering, a mounting error, and the like, of each of the motor and drive gears for driving the intermediate transfer member.
Further, causes of the image defects include mutual interference caused by friction between the surface of the photosensitive drum and the transfer surface of the intermediate transfer member. This is caused because a speed variation occurring in one of the photosensitive drum and the intermediate transfer member has influence on the other.
As another cause, there may be mentioned an occurrence of an unplanned change in load on the intermediate transfer member during secondary transfer of a toner image carried on the intermediate transfer member onto a recording sheet, especially when the recording sheet is thick paper. This causes a high-frequency speed variation, and this speed variation causes positional displacement in the primary transfer.
As described above, there are various causes of the image defects, and it is very difficult to eliminate all of the causes.
To cope with this, there has been proposed a transfer mechanism configured such that an image transfer roller which corresponds to the intermediate transfer member causes an image roller which corresponds to the photosensitive drum to be friction-driven (see e.g. Japanese Patent Laid-Open Publication No. 2002-333752). The technique of this transfer mechanism is characterized in that images on the photosensitive drums become an image on the intermediate transfer member, and hence by forming the image on the intermediate transfer member with reference to respective positions on the photosensitive drums, the influence of irregular rotation of the photosensitive drums is reduced. As to this technique, it is important to form an image with reference to the rotational position of each photosensitive drum, and hence there has been proposed a method of performing exposure control in synchronism with an amount of rotational movement of the drum (see e.g. Japanese Patent Laid-Open Publication No. H08-99437). According to this method, it is possible to form electrostatic latent images on the photosensitive drums without positional displacement even if there is a variation in the amount of rotational movement of each photosensitive drum.
Further, the technique disclosed in Japanese Patent Laid-Open Publication No. 2002-333752 is further characterized in that even when the speed of the intermediate transfer member is varied e.g. due to an impact generated upon entrance of a recording sheet into a secondary transfer section of the intermediate transfer member, coincidence of respective images on the photosensitive drums and an image on the intermediate transfer member can be ensured, and hence image defects are less liable to be caused by the primary transfer.
However, in the method of shifting the timing of starting to form an electrostatic latent image between the photosensitive drums by a time period required for an image already transferred onto the intermediate transfer member to be conveyed from one photosensitive drum to the next one, so as to prevent occurrence of image color shift, there arises a problem that a time period required for a latent image and its resulting image formed by development on each photosensitive drum, to advance from an exposure light irradiated position to a primary transfer position, and a time period required for a transferred image on the intermediate transfer member to be conveyed from one photosensitive drum to another are made different by an extent corresponding to variation in the respective speeds of the photosensitive drum and the intermediate transfer member.